Signal transmission system



' l. G. WILSON SIGNAL TRANSMISSION SYSTEM July 23, 1940. 1

s Sheets-Sheet 1 Filed Feb. 11, 1959 lNVENTOR 6. WILSON 'AYAI. IAYIIA zo fi I ATTORNEY July 23, 1940. wlLSON 2,208,617

SIGNAL TRANSMISSION SYSTEM Filed Feb. 11, 1939 5 Sheets-Sheet 2 INVENTOR By I 6. WILSON A UNITED STATES Patented July 23, 1940 PATENT OFFICE SIGNAL TRANSMISSION SYSTEM Ira G. Wilson, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a. corporation of New York Application February 11, 1939, Serial No. 255,812

9 Claims.

This invention relates to signal transmission systems and particularly to pilot channels for carrier current systems.

One object of the invention is to provide a sig nal transmission line having pilot and carrier channels thereon and a main station with auxiliary stations on each side thereof that shall control amplifiers at said stations according to the pilot current at the main station to maintain the signal level constant at the main station and to maintain the signal level at the auxiliary stations beyond the main station approximately constant.

Another object of the invention is to provide a cable comprising a number of pairs of conductors having pilot and carrier channelsthereon and a main station with auxiliary stations on each side of the main station that shall control amplifiers at said stations on each pair of conductors according to the pilot currents on a number of pilot channels at the main station to'maintain the signal level on the pairs of conductors constant at the main station and to maintainthe signal level on pairs of conductors at auxiliary stations beyond the main station approximately constant. a

A further object of the invention is to provide :a signal transmission line having pilot and car- .rier channels thereon and a main station with maintain the energy level of the signals. The vgain effected at the repeater stations must be governed according to the losses on the line in order to maintain the energy level of the signals constant.

In one form of the invention a signal transmission line carrying pilot and carrier channels has a main station and auxiliary stationslocated on each side of the main station. Amplifiers of the feedback type are located at each of the stations. The amplifiers are each controlled by-an element having a high negative temperature coefiicient of resistance, for example, silver sulphide inserted in sthe beta circuit. At the main station pilot current on the pilot channel is filtered off and employed for controlling a low frequency source of current. The low frequency current which is controlled according to the energy level of the pilot current is employed to energize heating coils associated with the resistance elements in the beta circuits of the various amplifiers.

The auxiliary stations on the transmission line before the main station are controlled by a backward acting circuit and will insure a constant energy'level for the signals at the main station. The'auxiliary stations on the transmission line beyond the main station are controlled by a forward acting circuit. A forward acting control circuit in a pilot channel system will not be able to exactly maintain the energy level of the signals constant at the controlled stations. However, it is possible to maintain the energy level of the signals approximately constant and rel ieve the work required at the next main station. 'It is apparent that the next main station will holdthe energy level of the signals substantially constant.

In a pilot channel arrangement constructed as above indicated it is apparent that a saving will befitedin the amount of apparatus required and that the signal level will be held close to the level required. At the terminal of the transmission line means will be provided by mop-up circuits to maintain the energy level of the signals constant. In themop-up circuits no forward acting control circuits will be employed In a cable carrier system constructed in accordance with the invention the pairs of conductors in the cable will have not only carrier channels but also pilot channels. A main station on the cable will have a number of auxiliary stations located on each side thereof. An ampliffier is provided at each station for each pair of conductors. The amplifiers are preferably of the feedback type controlled by a resistance element having a high negative temperature coefficient of resistance. The resistance elements which are located in the beta circuits of the amplifiers are controlled by heater coils.

A number of the pilot currents on the pilot channels are selected at the main station and combined for governing a low frequency control current. A number of pilot currents are employed in order to insure against failure of the gain control apparatus in case of failure of one pilot current, The controlled low frequency current is employed to govern the amplifiers on the stations before the main station by means of backward acting circuits and to control the amplifiers H and plifiers' at the stations beyond the main station by forward acting circuits. The energy level of the signals on the different pairs of conductors will be maintained substantially constant at the main station. At the auxiliary stations beyond the main station the energy level of thesignals can be maintained only approximately constant.

In the accompanying drawings,

Figs. 1, 2 and 3 taken together disclose a gain control circuit constructed in accordance with the invention. In assembling the drawings, Fig. 2 is attached to the left side of Fig. l and Fig. 3 is attached to the left side of Fig. 2.;

Figse, 5 and 6 taken together disclose a carrier cable system constructed in-accordancewith the invention. In completing the circuit shown in Figs. 4, 5 and 6, Fig. 5 is attached to the left side of Fig, 4 and Fig. 6 is attachedto the-.left side of Fig. 5.

Referring to Figs. 1, 2 and 3 a carrier current transmission systemis illustratedicomprising,two conductors. and 2. A main station;3 and four auxiliary stations 4,5, Stand ,1 are connected to .the transmission line comprising conductors l and 2. Theauxiliary stations 4 and .5 are positioned at one side-of the main station 3 and the auxiliary stations .6 and 1 are positioned on the other side ofv the main station 3.

An amplifier 8 in the transmission line at the main station 3 iscontrolled by a beta circuit having aresistance element 9 therein. The resistance element 9 has a high negative temperature coefficient of resistance and preferably is composed of silver sulphide. The resistance element is controlled by heater coil l0. Amplifiers H and I2 are respectively placed in the transmission line at the auxiliary stations 5 and 4. The amplifiers I and I2 have beta circuits which are respectively controlled by resistance elements l3 and |4.. Resistanceelements l3 and M are of similar construction to ,the resistance elements 9 at station 3. .Heater coils I5 and I6 are respectively provided for controlling the resistance elements i3 and l4.

Amplifiers I1 and |8 are positioned in the transmission line at stations 6 and I. The am- Hi are provided with beta circuits respectively controlled by resistance elements 9 and 20. Resistance elements l9 and 20 have high negative temperature coefilcients of resistance and preferably are composed of silver sulphide. Heater coils 2| and 22 are respectively provided for controlling the resistance elements I9 and 20.

The transmission line comprising conductors and 2 not only has a number of carrier chan- .n-, el s thereon for.signal purposes but has a pilot channel supplied with pilot current by means of a suitable source 23. The carrier currents and the pilot current may have relatively high frequencies. At the main station 3 a filter 24 vis provided for selecting the pilot current transmitted over the line from the source 23. The pilot current selected by the filter 24 is amplified by amplifier 25 and rectified by a rectifier doubler 26. The rectified pilot current supplied by the rectifier doub1er26 is supplied to a control tube 2'|. The plate circuit of the tube 21 is connected to a source 28 of relatively low frequency current so that the low frequency current from the source 28 is controlled in accordance with the amplitude of the pilot current on the transmission line at station 3. The controlled low frequency current from the control tube 21 is amplified by tube 29 and supplied to two low fre- "quency control conductors 39 and-3|. Inthe circuit shown inFig. 2 of the drawings the filter 24 is connected to the amplifier 25 by means of a transformer 32. The amplifier 25 is connected to the rectifier doubler 26 by means of a transformer 33. The control tube 21 is connected to amplifier tube 29 by means of transformer 34. The amplifier 29 is connected to the conductors 30 and 3| by means of a transformer 35. The conductors 3B and 3| supply low frequency control current to each of the stations on the transmission line for controlling the gain by the various amplifiers.

The heater coil II] for controlling the resistance element 9 at station 3 is connected to the control conductors 39 and 3| by means of a transformer 36. Adjustable resistance elements 31 and 38 are provided in the connections between the heater coil In and the transformer 36 for adjusting not only the rate of gain but the flat gain effected by the amplifier 8. In the transmission line under consideration it isassumed that the various sections of the transmission line are subjected to substantially the same temperature conditions so that the fiat gain effected at a stationmay be adjusted as above set forth.

Heater coils l5 and I6 at stations 5 and 4 are respectively connected to the control conductors 3|) and 3| by means of transformers 39 and 40.

Adjustable resistance elements 4| and 42 are pro- I vided in the connection between the heater coil I5 and the transformer 39. Adjustable resistance elements 43 and 44 are provided in connec tion between the heater coil l6 and the transformer 49. The adjustableelements 4| and 42 at station 5 and adjustable resistance elements 43 and 44 at station 4 serve to adjust the rate of gain and the flat gain at the stations 5 and 4. j

The heater coils 2| and 20 at stations 6 and l are respectively connected to the control conductors 3D and 3| by means of transformers 45 and 46. are provided in the connections between the heater coil 2| and the transformer 45 and adjustable resistance elements 49 and 50 are provided in the connections between the heater coil 22 and the transformer 45. Resistance elements '41 to 53, inclusive, at stations 6 and I serve to adjust the rate of gain and the flat gain efiected by the amplifiers H and I8 at stations 6 and The pilot current supply from a source 23 is selected by the filter 24 for governing the operation, of the amplifiers at the stations 3, 4, 5, 6, and-l. The selected pilot current is amplified by the amplifier tube 25, rectified by the rectifier doubler 26 and supplied to control tube-21 for governing thelow frequency current from the source 28. The controlled low frequency current is amplified by the amplifier 29 and supplied to the control conductors 30 and 3|. The control current is fed back for controlling stations 4 and 5 and is fed forward for controlling stations 6 and I. The heater coils l5. l5 and it] at stations 5, 4 and 3 are controlled bythe low frequency current on the conductors 30 and 3| to maintain the energy level of the-signals on transmission line at station '3 substantially constant.

At stations 6 and I the energy level of the signals can only beheldapproximately constant. By means of a forward acting circuit the ampliit is possible to relieve. the load-carried by th :7

Adjustable resistance elements 41 and 48 fiers at stations-6 and'l can be controlled to ap-' nextmain station. It should benoted in the system shown in Figs. 1, 2 and 3 only one filter is connected to the transmission line and the only connection between thevarious auxiliary stations and the main station is a circuit carrying the low frequency control current. Theapparatus for adjusting the-low frequencycurrent according to the energy level of the pilot current is only necessary at the main station. In a system constructed as above set forth it may be .noted the distance through which the control currents are transmitted is reduced by one-half. This greatly reduces the line loss by'the control currents and therefore the voltage required by the control currents. l

Referring to Figs. 4,5 and 6, a carrier cable system is illustrated comprising a number of pairs of conductors having not only carrier channels but also pilot channels. In Figs. 4, 5, and 6 five pairs of conductors 52, 53, 54 and 55 are illustrated. It is to be understood, however, that the cable may be provided with any desired number of similar pairs of conductors.

One main station 56 and four auxiliary stations.

51, 58, 59, and 60 are positioned on the pairs of conductors 5| to 55, inclusive. The stations '51 and 58 are positioned on oneside of the station 56 and the stations 59 and 60 are positioned on the other side of the main station 56. Amplifiers 6| are provided on the pairs of conductors 5| to 55,-inclusive, at the main station 56. The amplifiers 6| each have a beta circuit controlled by a resistance element 62. The resistance elements 62 are controlled by heater coils 63. Am-

plifiers64 are. provided at station 51 for the pairs of conductors 5| to 55, inclusive.

controlled by heater coils 69. Amplifiers 16 are provided at station 59for the pairs of conductors 5| to 55, inclusive. Beta circuits for the amplifier are controlled by resistance elements 1|. Resistance elements 1| in turn are controlled by heater coils I2. Amplifiers 13 at station 60 are provided for the pairs of conductors 5| to 55, inclusive. The amplifiers 13 are provided with beta circuits under control of resistance elements M. Resistance elements 14, the same as resistance elements H, have a high negative temperature coefficient of resistance and may be composed of silver sulphide. The resistance elements 14 are controlled by heater coils 1'5.

A pilot filter I6 is connected to the pair of conductors 54 at station 56 to select a pilot curformer 8| and the output of the amplifier 80 is connected by the transformer 82 to a rectifier doubler 83. The filter 18 is connected by a transformer 84 to an amplifier 85. The output of the Beta circuits for the amplifiers 64' are controlled by amplifier 85 is connected by a transformer 86 to the rectifier doubler 83. The rectifierdoubler fl3sis connected to a control tube 81. A source of low frequency current 88 is connected to the output circuit of the tube 81!. The tube 8'! is connected by transformer 89 to an amplifier 90 and the amplifier 90 is connected by transformer 9| to a pair of control conductors 92 and 93.

The two pilot currents selectedfrom the pairs of conductors 54, and 55 jointly control the rectifier doubler 83. The rectified pilot currents govern the control tube 81 to control the low frequency current from the source 88 according to the amplitudes of the pilot currents on the 75- at stations 59 and 6|] are controlled by a forard acting circuit. Adjustable resistance elements 94 are associated with the heater 'coil 56 for controlling the rate of gain and the flat gain of the amplifiers 54. Adjustable resistance elements 95 are associated with the heater coil 69 -for'controlling the rate of gain and the flat gain of the amplifiers 67. Adjustable resistance elements 96 are associated with the heater coil 63 for controllingtheirate of gain and the flat I gain of the amplifiers 6|. Adjustable resistance elements '9! and 98 are respectively associated with the heater coils 12 and 15 for controlling the rate of gain and the fiat gain of the amplifiers 10 and 13.

The control current on the conductors 92 and 93'which is governed in accordance with the pilot currents on the pairs of conductors 54 and control the amplifiers at the stations 57, 58 and 56 to maintain the energy level of the signals at the main station 56 substantially constant. The control current on the conductors 92. and

. .93 governs amplifiers at stations 59 and to maintain the energy level of the signals at these stations substantially constant.

Modifications in the systems and in the arrangement and location of parts may be made within the spirit and scope of the invention and such modifications areintended to be covered by the appended claims.

What is claimed is:

1. 'In combination, a signal transmission line having pilot and carrier channels thereon, said channels carrying pilot and carrier currents, a main station and a plurality of auxiliary stations on said line, said auxiliary stations being located on each side of said main station, an amplifier for said channels at each of said stations, and means controlled by pilot current at the main station for simultaneously governing each of said amplifiers to maintain the level of the currents on the channels constant at the main station and at the auxiliary stations before said main station and to maintain the level of the current at the auxiliary stations beyond said main station approximately constant.

2. In combination, a signal transmission line having'a pilot channel and a number of carrier channels thereon, a main station and a plurality of auxiliary stations on said line, said auxiliary stations being located on each side of said main station, an amplifier for said channels at each tions on said line, said auxiliary stations being located on each side of said main station, a source v of relatively low frequency current at said main 15 station, an amplifier for said channels at each of said stations, means for combining said pilot currents to-control the current from said low frequency source, and means for simultaneously controlling said amplifiers by said controlled low frequency current to maintain the signal level constant at themain station and at the auxiliary-stations before said main station and to maintain the level of the signals at the auxiliary stations beyond said main station approximately correct. I v

j 4. In combination, a cable having a plurality of pairsof conductors, a pilot channel and carrier channels onsaid pairs of conductors, a main station and a plurality of auxiliary stations. on

said cable, an amplifier for the channels on each,

of said pairs of conductors at each station, and means controlled by the pilot current, on thepilot channel atthe main station for simultaneously governing each of said amplifiers to maintain the signal level constant at the main station and at the auxiliary stations before said'main station and to maintain the level of the signals at the auxiliary stations beyond said main station approximately constant.

'5. In combination, a cable having a plurality of pairs of conductors, pilot channels and carrier channels on said pairs of conductors, an amplifier for the channels on each. of said pairs of conductors, 'means for picking a number of pilot currents from the pilot channels and means for combining said pilot currents and for controlling said amplifiers according to the strength .of said pilot currents.

6. In combination, a plurality of pairs of conductors, pilot channels and carrier channels on said pairs of conductors, a station having amplifiers respectively associated with said pairs of conductors, means for filtering ofi the pilot cur rentsefrom two pilot.channels, and means-for combiningsaid: pilot currents and for controlling the am-plifiers at the station by said pilot currents. l

-7. In combination, a plurality of pairs of conductors, pilot channels and carrier channels on 8. In combination, a cable having a plurality of pairs of conductors, pilot channels and carrierchannels on said-pairs of conductors, a main station and auxiliary stations for said cable, said auxiliary stations being located on each side of said main station, means for picking two pilot currents from the pilot channels at the main station and means for combining said two pilot currents and for simultaneously controlling said amplifiers-according to the strength of said pilot currents to maintain the carrier current level constant on the different pairs of conductors at the main station and at the auxiliary stations before the main station and to maintain the level of the carrier currents approximately constant at the auxiliary stations beyond said main station.

9. In combination, a plurality of pairs of conductors, pilot channels and carrier channels on said pairs of conductors, a main station and auxiliary stations for ,said pairs of conductors,

said auxiliary stations being located on each side of said main station, an amplifier for said channels on each ofsaid pairs of conductors at each of said stations, a source of relatively low frequency current at said main station, means for picking offtwo pilot currents from the pilot.

channels at the main station, means for combining said pilot currents to control the current from said low frequency source and means for simultaneously controlling said amplifiers by said controlled low frequency current to maintain the signal level constant on the pairs of conductors at the main station and at the auxiliary stations before the main station and to maintain the level of the signals at the auxiliary stations beyond the main station approximately constant.

IRA G. WILSON. 

